You can reset the password buy pressing the "reset switch" on the back of the router. Press it for 30 seconds when you release it the router will be reset to the factory defaults. Google "router default passwords" or check you router instructions for the default ID and password. You will then need to set network ID and security password by logging on to the router using a wired connection to your PC.

Another way is to use this program to display the password from a working wireless connected PC.